EGSA Generator and Alternator Principles 2 ā Questions and Answers
Question 1: What is the primary function of slip rings in an AC alternator?
- Rectify AC to DC output
- Transfer current to the rotating field winding (Correct answer)
- Regulate voltage output
- Reduce armature reaction
Correct answer: Transfer current to the rotating field winding
Slip rings provide a continuous electrical connection to the rotating field winding, supplying DC excitation current without interruption.
Question 2: In a synchronous generator, what happens to the output frequency if the rotor speed increases by 10%?
- Frequency decreases by 10%
- Frequency remains the same due to AVR
- Frequency increases by 10% (Correct answer)
- Frequency doubles
Correct answer: Frequency increases by 10%
Output frequency is directly proportional to rotor speed (f = PĆN/120), so a 10% speed increase produces a 10% frequency increase.
Question 3: Which type of rotor is most commonly used in high-speed steam turbine-driven generators?
- Salient pole rotor
- Cylindrical (round) rotor (Correct answer)
- Claw pole rotor
- Wound rotor
Correct answer: Cylindrical (round) rotor
Cylindrical rotors are used in high-speed (3600 RPM) turbo-generators because their smooth surface minimizes windage losses and mechanical stress.
Question 4: What is meant by 'armature reaction' in a synchronous generator?
- Voltage drop across the armature resistance
- Effect of armature current magnetomotive force on the main field flux (Correct answer)
- Mechanical reaction torque on the stator
- Thermal expansion of armature windings
Correct answer: Effect of armature current magnetomotive force on the main field flux
Armature reaction is the distortion and weakening of the main magnetic field caused by the magnetomotive force produced by load current in the armature windings.
Question 5: A 4-pole alternator must rotate at what speed (RPM) to produce 60 Hz output?
- 1800 RPM (Correct answer)
- 3600 RPM
- 1200 RPM
- 900 RPM
Correct answer: 1800 RPM
Using f = PĆN/120: N = 120Ć60/4 = 1800 RPM for a 4-pole machine at 60 Hz.
Question 6: What is the purpose of damper windings (amortisseur windings) in a synchronous generator?
- Increase field excitation during load surges
- Dampen rotor oscillations and suppress hunting (Correct answer)
- Provide backup excitation if the AVR fails
- Reduce harmonic distortion in output voltage
Correct answer: Dampen rotor oscillations and suppress hunting
Damper windings are short-circuited bars embedded in rotor pole faces that produce braking torque to suppress rotor hunting and oscillations.
Question 7: How does increasing field excitation beyond the unity power factor point affect a synchronous generator connected to an infinite bus?
- It increases real power output
- It causes the generator to absorb reactive power (under-excitation)
- It causes the generator to supply reactive power (over-excitation) (Correct answer)
- It reduces the terminal voltage
Correct answer: It causes the generator to supply reactive power (over-excitation)
Over-excitation in a synchronous generator causes it to supply reactive power (act as a capacitor) to the bus, boosting system voltage.
EGSA Generator and Alternator Principles 2
What is the primary function of slip rings in an AC alternator?